It's a a propellantless microwave thruster that defies Newton's laws of motion

Chinese scientists have taken on a heavily criticized space drive idea that could one day launch satellites, deep space probes and even flying cars.

The research team hails from Northwestern Polytechnic University in Xi'an, and was led by Yang Juan. What they developed was a propellantless microwave thruster called EmDrive, which is a controversial idea because it goes against Newton's laws of motion -- and many have claimed to create a "propellantless" thruster before and failed.

Newton's laws of motion are all based on the idea that firing propellant out of the back at a high speed will push a craft forward. While solar cells offer infinite power, thrust is limited by propellant. Many have tried to get around this, but several scams have made this particular field a joke in the scientific world.

While space drives tend to rely on Newton's laws of motion, the EmDrive is a closed, conical container that has a net thrust toward the wide end when filled with resonating microwaves. This goes against Newton, who said that no closed system could have a net thrust. However, EmDrive works because the microwaves have a group velocity (the speed of a collection of electromagnetic waves) that is greater in one direction than the other -- which is where Albert Einstein's theory of relativity comes in.

British engineer Robert Shaywer, who began looking at the concept of a propellantless thruster when he opened his own company called Satellite Propulsion Research in 2001, is the original creator of the EmDrive. He made demonstration thrusters to prove it could be done, and even made sure the test results were accurate (meaning, the results weren't affected by friction, ionization, air currents, electromagnetic effects or interference). The first, made in 2003, had a thrust of 16 mN. This was enough to show it could be done.

However, he received a ton of criticism for his idea and was ridiculed in his own country. But the Chinese team at Northwestern Polytechnic University believed in his research and took the project head on to author the latest study, "Net Thrust Measurement of Propellantless Microwave Thruster."

The Northwestern Polytechnic University team was able to create the EmDrive with 720 mN of thrust with a couple of kilowatts of power.

So what purpose does EmDrive have? It could halve launch costs of satellites because as much as half the launch weight of these objects are attributed to propellant.

Shawyer is even working on a superconducting thruster that could be ready as soon as 2016. It would boost the Q value of the cavity, which determines the amount of thrust produced. He said it could be boosted by a factor of several thousand, possibly equating to a tonne of thrust per kilowatt of power.

I'm not sure I see how. If the device was able to produce thrust without any energy input, then yes, I can see how that would violate conservation of energy. However, this thruster requires a constant energy input. That energy must be generated from some fuel source, either through nuclear decay in a radioisotope generator or by capturing photons in a solar collector and converting them to energy. In either case your energy source is converting mass to energy to produce the electricity that the engine uses.

That energy is then dumped into the "reaction" chamber, producing thrust. I assume some (most) of that energy is wasted heating the chamber walls in a relatively even fashion, causing waste heat to dissipate in relatively equal amounts in all directions and producing no net effect. And then I further assume that the remainder of the energy is dissipated in a directional manner, producing useful thrust.

I don't see how any fundamental principal is being violated there; energy is being expended to perform work by producing thrust. Although the thrust doesn't involve ejecting massive particles out of the vehicle, it is not massless either; it has (*must* have, in fact, due to conservation of mass and mass-energy equivalence) a net mass equivalent to the matter that was destroyed to provide the electrical energy used to drive the engine, and it departs the vehicle at the speed of light.

I don't see how this is fundamentally very different than the ion drive concept. The only change is that instead of converting a small amount of matter to energy and then using that energy to cause another small piece of matter to leave the vehicle and produce thrust; the middle-man is removed and the energy itself provides the thrust. It seems like it's basically equivalent to ejecting tiny bits of your nuclear fuel-source out the rear of the craft at the speed of light as it decays, instead of converting those tiny bits of matter to electricity.

The violated part of physics is the realation between impulse and energy. For a photon, regardless of its wavelength, this ratio is defined by the speed of ligth, i.e. p = E/c. In order to generate a thrust of 720 mN for the drive, you need to emit every second a packet of energy carriers with an impulse of 720mN*1s. If this is done with photons only, those photons have a corresponding energy of 720mN*1s*300,000,000m/s = 216 MJ per second, i.e. 261MW. Not "a few kilowatt" as discussed in this article. And it makes no difference how often you bounce these photons around in a cavity, they will never gain any additional impulse.

A fuel-based propulsion system on the other hand follows the relationship p=2E/v, which gives you significantly better Energy to Thrust ratios, as long as the velocity of the fuel is much smaller than the speed of light. Sadly, energy efficiency is best for a huge mass that is exhausted slowly, but that means a horrible fuel efficiency. And usually in space, energy is cheap, but mass is not. Thats why deep space vehicles are now often equipped with ion thrusters, which need a lot of energy but only very little fuel.

The photons do not cease to exist. Only a tiny fraction of the photon's energy is converted to kinetic energy. The impulse is the integral of force with respect to time. A force of 720 mN applied for 1 second is then an impulse of 0.72 N-s (Newton-seconds), which is equivalent to the change in momentum. A 0.72 kg mass would experience a 1 m/s^2 acceleration. So applied for 1 second, the change in kinetic energy would be 0.5*(0.72 kg)*((1.0 m/s)^2 = 0.36 J = 360 mW/s.

This very low efficiency (0.018%), doesn't look good at first glance. But let's say we have a spacecraft with a mass of 720 kg, 320 kg of which is a TOPAZ-I nuclear reactor generating 5 kW sustained for 3 to 5 years. Dedicating 4 kW to propulsion allows an impulse of 1.44 N-s, and so an acceleration of a paltry 0.002 m/s^2. However, this small force can be applied for years. In one week the spacecraft will have accelerated to 1.2 km/s. In 6 months it will be traveling at 30 km/s. In 3 years it will be traveling at nearly one third light speed.

With an ion drive, the conserved momentum is the directed (vector) sum of mass times velocity. The ions have a very low mass and a very high velocity. The spacecraft has a high mass and therefore receives a very small velocity increase per ion. The thrust force is the rate of change of momentum (Newton).

This new drive concept does not appear to have anything that balances the forward momentum gained by the spacecraft. Photons could provide a very small thrust force if they exited the engine. When they are trapped inside a cavity, they can't provide any net change in momentum.

If a bomb goes off in a very strong conical chamber, no net thrust occurs unless the chamber ruptures and lets high speed gas escape.

quote: This new drive concept does not appear to have anything that balances the forward momentum gained by the spacecraft. Photons could provide a very small thrust force if they exited the engine. When they are trapped inside a cavity, they can't provide any net change in momentum.

This applies if the only source of thrust comes from movement of mass. But take something like a rail gun, the projectile is not moved by any expulsion of mass but by interaction of the projectile with alternating magnetic fields. What if the resonate microwaves are causing thrust not through their mass equivalency but through their magnetic equivalency? Could the moving electromagnetic fields of the microwave photons be interacting with the Earth's magnetic fields in a way that appears to give thrust? Does anyone know for certain that a resonate microwave does not somehow interact with a gravitational field? Until we know more about gravity it can not be positively ruled out.

The recoil force pushes the rail gun backwards. If it were not attached to the earth, the rail gun would move backwards with a noticeable velocity. Otherwise, the rail gun and earth receive a tiny velocity contribution in the opposite direction of the projectile.

BTW, the recoil force is transmitted by the virtual photons of the magnetic field. Momentum is still conserved.